Many orthopaedic procedures involve the implantation of prosthetic devices to replace badly damaged or diseased bone tissue. Common orthopaedic procedures that involve prosthetic devices include total or partial hip, knee and shoulder replacement. For example, a hip replacement often involves a prosthetic femoral implant. The femoral implant usually includes a rigid stem that is secured within the natural femur bone tissue. The femoral implant further includes a rounded head that is received by, and may pivot within, a natural or artificial hip socket. Shoulder replacement is somewhat similar, and typically includes a humeral implant that includes a rigid stem and a rounded head. The rigid stem is secured within the natural humerus bone tissue and the rounded head is pivotally received by a shoulder socket.
Increasingly, prosthetic devices are provided as subcomponents that are assembled during surgery. In particular, the different anatomies of different patients require that prosthetic devices such as femoral and humeral implants be available in different sizes and configurations. By way of simplified example, a humeral implant may be available in as many as six or more humeral head diameters. Stems may similarly vary in size and/or in shape. Because of differences in patients and individual conditions, it is advantageous that the surgeon have at her disposal many configurations and sizes of implants. Instead of providing a separate implant for each possible combination of features, implants are provided as modular kits of subcomponents that allow the surgeon to mix and match different subcomponents to achieve the most advantageous combination for the patient. Thus, the surgeon can pick from several sizes or configurations of each component and combine the components to form an implant having an optimal combination of features.
One example of a modular implant is the humeral implant 10 shown in FIGS. 1 and 2. The humeral implant 10 includes a humeral head 12 that may be assembled onto a humeral stem 14. The humeral stem 14 is configured to be implanted in the intramedullary tissue of a natural humeral bone, while the humeral head 12 is configured to be received into the shoulder socket or glenoid cavity.
In the exemplary modular implant of FIGS. 1 and 2, an intermediate component 16 is provided between the humeral head 12 and the humeral stem 14. The intermediate component 16 is a two part insert that includes a stem insert 17 and a head insert 19. The stem insert 17 is provided within a cavity at the end of the stem 14. The head insert 19 includes a truncated ball portion 21 and a frusto-conical portion 23. The truncated ball portion 21 of the head insert is configured to fit within a receptacle in the stem insert 17. The frustro-conical portion 23 serves as a tapered plug 16 that is designed to be received by a tapered receptable 28 in the humeral head 12. It can be appreciated that the surgeon may secure alternative humeral head 12 designs on the same humeral stem 14, thus providing the surgeon with a broad array of humeral head size options.
Once the components are selected, such as the humeral head 12, the humeral stem 14, and the intermediate component 16 of FIGS. 1 and 2, the components are assembled. One popular method of securing implant components together involves the use of a Morse taper. The components of FIGS. 1 and 2 by way of example include a Morse taper arrangement. In particular, a Morse taper is a feature in which a tapered male component, e.g., the tapered plug 23 of the head insert 19, is received into a tapered female component, e.g., the receptacle 28 of the humeral head 12. The taper angle of the plug 23 is preferably, but need not be, slightly less than the taper angle of the receptacle 28. In use, the plug 23 advances into the receptacle 28, as indicated by arrow 29, until it begins to engage the receptacle 28. The further into the receptacle the plug 23 is forced, the more tightly it engages the humeral head 12.
The force applied to secure the plug 23 within the receptacle 28 is proportional to the retention force of the plug 23 within the receptacle 28. Thus, if a sufficient amount of force is applied, then the humeral head 12 will be securely fastened to the humeral stem 14 via the insert 16. Other prosthetic devices employ Morse tapers for substantially the same reasons.
To apply sufficient force to lock the Morse taper arrangement between the humeral head 12 and the plug 23, it is known to impact the humeral head 12 such that the impact force directs the humeral head 12 toward the plug 23 and humeral stem 14. The impact force drives the plug 23 into the receptacle 18 and forms the Morse taper lock. A hammer or mallet is typically struck directly on the head, or through an impactor device.
During assembly of the implant, the surgeon (or other person) may impact the prosthetic implant several times without knowing if sufficient force has been applied to lock the Morse taper sufficiently. In order to be sure that the Morse taper is locked, the surgeon or assistant may use excessive force. The use of excess force is undesirable because of the potential for damage to the bone tissue or the implant device. For example, the use of excess force may disengage the intermediate components between the head 12 and the stem 14, such as the insert components 17 and 19, from their locked position.
Thus, there is a need for assisting surgical personnel in applying the proper amount of force to a Morse taper to lock the Morse taper. In particular, it would be advantageous to provide an impactor device capable of dissipating the force that is transmitted through the impactor and to an implant when locking a Morse taper. Such an impactor would serve to limit the application of excessive force and any associated damage. The need for such a device is widespread as Morse tapers have commonly been used for connection of many types of implant devices. It would also be advantageous if such an impactor could be manufactured simply and at a low cost.